Non-volatile memory is a class of memory in which the memory cell or element does not lose its memory state after power supplied to the element is removed, and includes electrically erasable programmable read-only memory (EEPROM), flash memory, ferroelectric RAM (FeRAM), magnetic RAM (MRAM) to name but a few.
In flash memory, an ability to keep random access (erase/write single bits) was sacrificed for speed and higher bit density in comparison to EEPROMs. Flash memory remains a non-volatile memory of choice. Nevertheless, it is generally recognized that flash memory technology may not scale easily below 40 nanometers (nm).
Technologies considered for the replacement of, or for use in parallel with, flash memory have included resistance-based memory which is based on certain materials that exhibit a resistance change associated with a change of phase of the material (determined by the long range ordering of the atoms in the crystalline structure).
In one type of such resistance-based memory, called a phase change memory (PCM/PCRAM), a change in resistance occurs as the material of the memory element is melted briefly and then cooled to either a conductive crystalline state or an insulative amorphous state. However, resistance-based memories have not proved to be commercially useful because their transition between the conductive and the insulative states depends on a physical structure phenomenon, e.g. in a process of melting at up to 600° C. and returning to a solid state, whereby the process cannot be sufficiently controlled to be used as a reliable memory, and therefore is unsuitable for many applications.
Another resistance-based memory includes memory elements consisting of materials that respond to an initial high “forming” voltage and current to activate a variable resistance function. For example, the operation of resistive RAMs (ReRAMs) or conductive bridge RAMS (CBRAM) may be strongly temperature dependent, such that a resistive switching mechanism in a ReRAM/CBRAM may also be strongly temperature dependent. Certain types of ReRAM may also exhibit unstable qualities. Further, resistance switching in ReRAM/CBRAMs tends to fatigue over many memory cycles.